In Surah Az-Zumar (The Groups), Allah describes sleep as a state in which the soul is "taken":
"Allah takes the souls at the time of their death, and those that do not die [He takes] during their sleep. Then He keeps those for which He has decreed death and releases the others for a specified term. Indeed in that are signs for a people who give thought." (Az-Zumar 39:42)
This verse establishes a striking parallel between sleep and death, an analogy that the Prophet Muhammad ﷺ himself emphasized by calling sleep the "little death" (al-mawt al-asghar).
Sleep: a state of altered consciousness
Modern neuroscience has shown that sleep is not a simple cessation of brain activity, but a complex and active physiological state, characterized by a profound modification of consciousness.
During deep sleep (slow-wave sleep), brain activity slows considerably, sensory perception is largely disconnected from the outside world, and the brain enters a state of functioning very different from wakefulness. This "disconnection" from the external world presents analogies with certain states of reduced consciousness.
The discovery of REM sleep
In 1953, researchers Eugene Aserinsky and Nathaniel Kleitman of the University of Chicago discovered REM sleep, a phase during which the brain is very active while the body is paralyzed. This discovery revolutionized the understanding of sleep and showed that sleep is a far more complex phenomenon than a simple "loss of consciousness."
REM sleep, during which most dreams occur, illustrates the brain's ability to generate intense mental experiences while the body is completely inactive — a reality that resonates with the Quranic description of the soul that "travels" during sleep.
Sleep and memory
Neuroscience has also demonstrated the essential role of sleep in memory consolidation and the elimination of metabolic waste from the brain. The glymphatic system, recently discovered, is particularly active during deep sleep, cleaning the brain of toxic proteins accumulated during wakefulness.
This "restorative" function of sleep, essential to survival and proper cognitive functioning, confirms the wisdom of the wake-sleep alternation evoked in the Quran as a sign of divine mercy.
A sign for those who reflect
Verse 39:42 invites reflection on the subtle boundary between sleep and death. Modern science, by revealing the complexity of states of consciousness and the vital role of sleep, sheds new light on this Quranic analogy fourteen centuries old.